1. Field of the Invention
This invention relates to a cell search determination circuit for a mobile station in W-CDMA (Wideband-Code Division Multiple Access) cellular communications.
2. Description of the Related Art
Cell search in a W-CDMA system is an initial synchronization acquisition function to identify a scramble code number at a mobile station, wherein receiving timing is detected and a scramble code group is identified in a primary synchronization channel (PSCH) and a secondary synchronization channel (SSCH) for symbols without scramble codes (mask symbols), and then a scramble code number is identified in a common pilot channel (CPICH).
Hereinafter, a frame structure of each channel employed for cell search will be explained. FIG. 7 shows a structure of the PSCH and the SSCH. One frame, which is a collection of 15 slots, is 10 msec. A base station transmits one symbol of the PSCH and the SSCH at the head position of each slot, with a length of 256 chips per symbol at a fixed period.
In the PSCH, a common code acp is employed in the entire W-CDMA system. In the SSCH, sixteen kinds of codes are arranged in a certain arrangement acsnm at each slot. FIG. 8 illustrates an example of a table of the SSCH code number arrangement, in which n and m correspond to scramble code group numbers (Group 1 to 64) and slot numbers (#0 to 14) respectively. In the CPICH, known pilot patterns are transmitted at 15 kbps.
FIG. 9 shows a structure of a scramble code. For a scramble code in cell search, 512 primary codes 16×0, 16×1, . . . , 16×511 from Set #1 (0 to 15), Set #2 (16 to 31), . . . , Set #512 (8176 to 8189) each of which is a collection of 16 kinds of codes are employed. The 512 codes are divided into 64 groups with 8 codes per a group. Each scramble code group is arranged in the SSCH code pattern as shown in FIG. 8. Therefore, by detecting these codes, a mobile station can identify a scramble code group number and detect the receiving timing (frame timing) of a scramble code.
FIG. 10 is a flow chart of a known cell search method. At STEP I, slot timing is detected using the PSCH (101). STEP I will be now described.
FIG. 11 is a block diagram showing a structure of STEP I. A receiving signal is applied to a matched filter (MF) 801, wherein a correlation operation at each timing is sequentially performed. To the MF 801, diffusion code PSCH codes, which are commonly employed in W-CDMA systems, are applied. As shown in FIG. 7, at the PSCH, one symbol exists in a slot, so that the correlation operation of a receiving signal and PSCH codes in plural slot sections is performed, with the result that the self-correlation of the PSCH is outputted at a slot period. To reduce phase fluctuations, the output from the MF 801 is averaged at slot intervals in plural slot sections in a power adder 802. The average operation means a power addition average. With this average operation, receiving power, which results from self-correlation of the PSCH at a slot period, increases. In addition, the level ratio also increases, since noise components do not have any periodicity and they are suppressed. Slot timing detector 803 detects slot timing, which is necessary to perform a correlation operation of diffusion code SSCH codes at STEP II based on the average result of the power adder 802.
Next, at STEP II, a scramble code group is identified and frame timing is detected using the SSCH. Hereinafter, STEP II will be described.
FIG. 12 is a block diagram showing a structure of STEP II. A receiving signal is applied to a finger 901 having seventeen fingers. To each finger of the finger 901, the PSCH code and all of the sixteen kinds of SSCH codes (1 to 16) are applied. Both of the PSCH and SSCH are transmitted from a base station at a slot period, and the finger 901 performs a correlation operation at the slot timing detected in STEP I and outputs the self-correlation results of the PSCH and SSCH. In the SSCH, the SSCH code number arrangement is determined by scramble code group numbers and slot numbers, so that the self-correlation result of one of the sixteen kinds of SSCH codes is outputted. The other SSCH codes are considered as noise components.
The PSCH is employed for vector adjustment of the SSCH. To reduce phase fluctuations, as in the case of the STEP I, the output from the finger 901 is averaged at each frame in plural slot sections in a voltage adder 902. This average operation means a voltage addition average of I, Q phases in a complex I, Q plane. A comparator 903 compares the average result generated by the voltage adder 902 with the SSCH code number arrangement table shown in FIG. 8. In an identifier of group number 904, frame timing detection and identification of a scramble code group number, which are necessary for a CPICH correlation operation at STEP III, are performed.
At this point, a determination is made whether frame timing detection and identification of a scramble code group number could be performed (103). If the identification is not successful (103, NO), slot timing detection at STEP I is considered to be failed, returning to STEP I (101) to resume cell search. If the identification is successful (103, YES), the process proceeds to STEP III (104).
Next, at STEP III, a scramble code number is identified with the CPICH (104). STEP III will be now described.
FIG. 13 is a block diagram showing the configuration of STEP III. A receiving signal is applied to a finger 1001 having eight fingers. To each finger in the finger 1001, scramble code candidates (0 to 7) including all of the eight kinds of CPICH are applied. The finger 1001 performs the CPICH correlation operation at the frame timing detected at STEP II, and outputs a self-correlation result for one of the eight kinds of scramble code candidates. The other scramble code candidates are considered as noise components.
To reduce noise, as in the case of STEP II, outputs from the finger 1001 are averaged at each frame in plural slot sections in a voltage adder 1002. An identifier of scramble code number 1003 identifies the maximum value of the average results generated by the voltage adder 1002 as a scramble code number.
Next, to reduce missynchronization at STEP III, confirmation is performed with a threshold value using the CPICH (105). The confirmation with a threshold value will be now described.
FIG. 14 is a block diagram showing the configuration of a conventional cell search determination circuit. A receiving signal received in an antenna 401 and a CPICH code are applied to a finger 402. The finger 402 transmits a self-correlation output of the CPICH to a power adder 403 at a symbol period.
At a comparator 404, output from the power adder 403 is compared with the level of a threshold value. Then, the comparator 404 determines whether the output from the power adder 403 exceeds the threshold value (106). The threshold value employed here is a fixed value. If the output from the power adder 403 is below the threshold value (106, NO), identification of a scramble code is considered to be failed at STEP III. Then, the process proceeds to 107, where a determination is made whether continuous identifications of the scramble code number are failed (107), if not so (107, NO), the cell search is resumed from the identification of a scramble code at STEP III (104). If the continuous identifications of the scramble code number is failed (107, YES), detection of slot timing at STEP I is considered to be failed, returning to STEP I (101) to resume cell search. If the output from the power adder 403 exceeds the threshold value, cell search is completed. As described above, by repeating the process of the flow chart in FIG. 10 at a mobile station, a scramble code from a base station is confirmed.
However, in the conventional cell search determination circuit, the threshold value, which is employed for level comparing with the output from the power adder 403, is a fixed value in a comparator 404. Therefore, the comparator 404 cannot accommodate output fluctuations, which is generated by phase fluctuations, in the power adder 403, so that misdetection occurs. If the fail rate of confirmation increases, the cell search is performed again, increasing the cell search time. In addition, if misdetection occurs, a mobile station can not connect to a base station, leading to serious problems.